Electrochemically modified carbon material for lithium-ion battery

ABSTRACT

The present invention provides an anode material for a lithium-ion battery comprising a carbon particle having a particle size of 5 μm to 30 μm, and including defective portions on a surface of the carbon particle, the defective portions being grooves formed by cathodically exfoliating graphene layers from the carbon particle.

TECHNICAL FIELD

The present invention relates to an electrically modified carbonmaterial used as a negative electrode active (anode) material of alithium-ion battery.

BACKGROUND ART

A lithium-ion battery (sometimes LIB) is a member of rechargeable(secondary) battery types in which lithium ions move from a negativeelectrode to a positive electrode during discharge and back whencharging.

LIBs have become integral to modern day portable electronic devices suchas laptop computers and cellular phones. Moreover, with the advent ofelectric vehicles (EV) and plug-in hybrid electric vehicles (PHEV),there is a great demand for developing high energy density LIBs capableof long cruising distance of EVs and PHEVs.

Three kinds of carbons: graphite, soft carbon and hard carbon have beenused for commercial LIBs as a negative electrode active material.Graphite may intercalate up to a maximum of one lithium atom per sixcarbon atoms under ambient conditions. Many of soft carbons heat-treatedaround 1200° C. show a maximum reversible capacity of about 300 mAh/g.Some hard carbons can intercalate up to over one lithium atom per sixcarbon atoms. From earlier reports of 400 mAh/g of reversible capacity,improvement of reversible capacity without an increase of irreversiblecapacity has been attempted and over 500 mAh/g of reversible capacitywith a small irreversible capacity of about 60 mAh/g has been achieved.

Graphite is a three-dimensional ordered crystal. Soft carbon and hardcarbon are constructed with two-dimensional ordered graphene sheetswhich are randomly stacked, that are called as a ‘turbostratic’structure. Soft carbon is called as a graphitizing carbon because it canbe relatively easily graphitized by heat treatment over 2000° C. On theother hand, hard carbon is hardly to be graphitized, even at 3000° C.under ambient pressure, so it is called as a hardly-graphitizable ornon-graphitizing carbon. The raw material usually determines whethersuch a carbon is obtained under soft or hard condition.

Typical raw materials for soft carbon include petroleum pitch and coaltar pitch. Acenaphthylene can be used in the laboratory as a substitutefor pitch. Hard carbon is obtainable by heat treating thermosettingresins such as phenolic resin, and vegetable fibers such as coconutshell. Some carbon materials heat treated at about 800° C. or less havea large capacity, but their discharging potential is too high to be usedin current cells that the cell voltage will be lower than 3 V. Thelithium-doping mechanism of these carbon materials is different from themechanism under consideration here.

Furthermore, there is an increased demand for a high capacity as well ashigh lithiation rate capability with regard to negative electrode active(anode) materials for lithium ion batteries. In order to meet thedemand, attempts were made to use metals or elements, such as Si and Sn,which can make an alloy with lithium as the anode material. Such metalsor elements have a higher theoretical charge and discharge capacity thancarbonaceous materials. However, such metals or elements have seriouschanges in volume, accompanied with charging/discharging of lithium andresulting in metal based anode active materials to creak and pulverize.Thus, when charging/discharging cycles are repeated, the metal basedanode active materials show a sudden deterioration of capacity and ashorter cycle life.

SUMMARY OF THE INVENTION

In order to solve these problems, a new attempt has been made to improvecapacity and rate capability of carbon based materials (graphite, softcarbon, hard carbon) by means of electrochemical modification of carbonparticles to modify their morphology and crystal structure for highercapacity and better rate capability.

That is, one aspect of the present invention provides an anode materialfor a lithium-ion battery comprising a carbon particle having a particlesize of 5 μm to 30 μm, and including defective portions on a surface ofthe carbon particle, the defective portions including grooves formed bycathodically exfoliating graphene layers from the carbon particle.

Another aspect of the present invention provides a method for preparingan anode material including: soaking carbon particles having a particlesize of 5 μm to 30 μm into an electrolytic solution, the electrolyticsolution containing a cation intercalatable between graphene layers inthe carbon particles; cathodically exfoliating graphene layers from thecarbon particles to form defective portions on surfaces of the carbonparticles; and collecting the carbon particles after the cathodicallyexfoliating.

Still another aspect of the present invention provides a lithium ionbattery including the above anode material.

The aspect of the present invention can provide an anode material for alithium ion battery that is excellent in capacity and rate capability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: Schematic diagram of experimental set-up for preparingelectrochemically modified carbon materials;

FIG. 2: SEM images of electrochemically modified carbon particles ofsample 1;

FIG. 3: SEM images of carbon particles of comparative sample 1; and

FIG. 4: Rate capability comparison of electrochemical modified carbonparticles (sample 1), electrochemical modified carbon particles withheat treatment (sample 2) and soft carbon raw materials (comparativesample 1).

MODES FOR CARRYING OUT THE INVENTION

The invention will be now described herein with reference toillustrative embodiments.

The present invention proposes to activate the basal and edge site ofcarbon materials with many defective portions, such as grooves, and thelike by cathodic exfoliation method. Reaction sites and Li-ion pathwayscan be increased after the activation, so as to increase the capacityand rate capability.

The raw carbon materials can be selected from particles includinggraphene layers such as nature graphite, artificial graphite,meso-carbon micro-bead (MCMB), graphitic coke, meso-phase carbon, hardcarbon, soft carbon, polymeric carbon, carbon or graphite fibersegments, carbon nano-fiber for graphitic nano-fiber, carbon nano-tube,or a combination thereof. Among them, the soft carbon particles arepreferable. The soft carbon particles are obtained from a precursor suchas petroleum pitches, coal pitches, oils such as low molecular heavyoils and meso-phase pitches. The carbon particles have an averageparticle size of 5 μm to 30 μm.

There is no special restriction for a width of the grooves. The capacityand rate capability vary with groove width. The groove width ispreferably distributed between 100 nm and 500 nm, inclusive. In thepresent specification, the groove width means an opening width. Theshape and distribution of the grooves depend on the distribution of thegraphene layers in the raw carbon materials.

The electrochemically modified carbon material can be prepared bycathodically exfoliating graphene layers from the carbon particles in anelectrolytic solution containing a cation intercalatable betweengraphene layers. For example, FIG. 1 shows a schematic diagram ofexperimental set-up for preparing electrochemically modified carbonmaterials. Carbon materials 1 are soaked in electrolytic solution 2 andthe electrochemical exfoliation is performed by using working electrode3 in contact with the soaked carbon materials 1 and counter electrode 4with applying predetermined negative potential from power source 5. Asfor the electrolytic solution, but not restricted, aqueous solutionincluding an intercalatable cation can be used. The intercalatablecation is preferably larger than the interlayer distance of the graphenelayers. Examples of such an intercalatable cation include a solvate ofalkaline metal cation with an organic solvent, particularly a solvate ofsodium ion with dimethyl sulfoxide (DMSO). For example, sodium chloride(NaCl) is mixed with DMSO with a volume ratio of 1:50 to 1:1 to obtain adesired solvate. The electrolytic solution is preferably prepared bymixing DMSO and water with a mixing ratio of DMSO/water of 10:1 to 1:1by volume and then adding NaCl with a concentration of 0.1 to 15 M/L. Inaddition, ionic liquid can be used as the electrolytic solution as itis. The ionic liquid is preferably liquid at normal temperature.Examples of the ionic liquid include imidazolium salts, pyridiniumsalts, pyrrolidinium salts, piperidinium salts, sulfonium salts,phosphonium salts, ammonium salts and mixture thereof.

The carbon particles are collected and washed to remove the electrolyticsolution and exfoliated graphene layers. The collected carbon particlescan be further heat treated under inert atmosphere such as argon,nitrogen or the like. The heat treatment can be carried out at atemperature range of 600° C. to 1400° C., preferably 800° C. to 1200° C.for 0.5 to 24 hours.

The electrochemically modified carbon material preferably furtherincludes anode active particles which are capable of absorbing anddesorbing lithium ions. Examples of the anode active particles include:(a) silicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony (Sb),bismuth (Bi), zinc (Zn), aluminum (Al), titanium (Ti), nickel (Ni),cobalt (Co), and cadmium (Cd); (b) alloys or intermetallic compounds ofSi, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Ni, Co, or Cd with other elements,wherein the alloys or intermetallic compounds are stoichiometric ornonstoichiometric; (c) oxides, carbides, nitrides, sulfides, phosphides,selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Fe, Ti, Ni,Co, or Cd, and their mixtures or composites; and (d) combinationsthereof. There is essentially no constraint on the type and nature ofthe anode active particles that can be used in practicing the presentinvention. Among them, metal or semi-metal particles or compoundparticles of at least one element selected from a group consisting ofSi, Sn, Al, Ge and Pb are preferable.

The electrochemically modified carbon material can be coated with a thinlayer of carbon after combining with active materials, such as Si, Sn,etc. For instance, micron-, sub-micron-, or nano-scaled particles orrods, such as SnO₂ nano particles, may be decorated on the surface ofelectrochemically modified carbon material to form a composite material.Then the composite material may be coated with carbon by pyrolysis(including hydrothermal synthesis) of hydrocarbons such as saccharidesor using CVD method.

Another exemplary embodiment relates to a lithium-ion battery includinga negative electrode comprising the anode material according to theabove exemplary embodiment. The battery also comprises a positiveelectrode comprising an active material, an electrolyte comprising alithium salt dissolved in at least one non-aqueous solvent and aseparator configured to allow electrolyte and lithium ions to flowbetween a first side of the separator and an opposite second side of theseparator.

As for the positive electrode active material, but there is also noparticular restriction on the type or nature thereof, known cathodematerials can be used for practicing the present invention. The cathodematerials may be at least one material selected from the groupconsisting of lithium cobalt oxide, lithium nickel oxide, lithiummanganese oxide, lithium vanadium oxide, lithium-mixed metal oxide,lithium iron phosphate, lithium manganese phosphate, lithium vanadiumphosphate, lithium mixed metal phosphates, metal sulfides, andcombinations thereof. The positive electrode active material may also beat least one compound selected from chalcogen compounds, such astitanium disulfate or molybdenum disulfate. More preferred are lithiumcobalt oxide (e.g., Li_(x)CoO₂ where 0.8≦x≦1), lithium nickel oxide(e.g., LiNiO₂) and lithium manganese oxide (e.g., LiMn₂O₄ and LiMnO₂)because these oxides provide a high cell voltage. Lithium iron phosphateis also preferred due to its safety feature and low cost. All thesecathode materials can be prepared in the form of a fine powder,nano-wire, nano-rod, nano-fiber, or nano-tube. They can be readily mixedwith an additional conductor such as acetylene black, carbon black, andultra-fine graphite particles.

For the preparation of an electrode, a binder can be used. Examples ofthe binder include polytetrafluoroethylene (PTFE), polyvinylidenefluoride (PVDF), ethylene propylenediene copolymer (EPDM), orstyrene-butadiene rubber (SBR). The positive and negative electrodes canbe formed on a current collector such as copper foil for the negativeelectrode and aluminum or nickel foil for the positive electrode.However, there is no particularly significant restriction on the type ofthe current collector, provided that the collector can smoothly pathcurrent and have relatively high corrosion resistance. The positive andnegative electrodes can be stacked with interposing a separatortherebetween. The separator can be selected from a synthetic resinnonwoven fabric, porous polyethylene film, porous polypropylene film, orporous PTFE film.

A wide range of electrolytes can be used for manufacturing the cell.Most preferred are non-aqueous and polymer gel electrolytes althoughother types can be used. The non-aqueous electrolyte to be employedherein may be produced by dissolving an electrolyte (salt) in anon-aqueous solvent. Any known non-aqueous solvent which has beenemployed as a solvent for a lithium secondary battery can be employed. Amixed solvent comprising ethylene carbonate (EC) and at least one kindof non-aqueous solvent whose melting point is lower than that ofethylene carbonate and whose donor number is 18 or less (hereinafterreferred to as a second solvent) may be preferably employed as thenon-aqueous solvent. This non-aqueous solvent is advantageous in that itis (a) stable against a negative electrode containing a carbonaceousmaterial well developed in graphite structure; (b) effective insuppressing the reductive or oxidative decomposition of electrolyte; and(c) high in conductivity. A non-aqueous solvent solely composed ofethylene carbonate (EC) is advantageous in that it is relatively stableagainst decomposition through a reduction by a graphitized carbonaceousmaterial. However, the melting point of EC is relatively high, 39-40°C., and the viscosity thereof is relatively high, so that theconductivity thereof is low, thus making EC alone unsuited for use as asecondary battery electrolyte to be operated at room temperature orlower. The second solvent to be used in the mixed solvent with ECfunctions to make the viscosity of the mixed solvent lowering than thatof which EC is used alone, thereby improving an ion conductivity of themixed solvent. Furthermore, when the second solvent having a donornumber of 18 or less (the donor number of ethylene carbonate is 16.4) isemployed, the aforementioned ethylene carbonate can be easily andselectively solvated with lithium ion, so that the reduction reaction ofthe second solvent with the carbonaceous material well developed ingraphitization is assumed to be suppressed. Further, when the donornumber of the second solvent is controlled to not more than 18, theoxidative decomposition potential to the lithium electrode can be easilyincreased to 4 V or more, so that it is possible to manufacture alithium secondary battery of high voltage. Preferable second solventsare dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), diethylcarbonate (DEC), ethyl propionate, methyl propionate, propylenecarbonate (PC), γ-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate(EA), propyl formate (PF), methyl formate (MF), toluene, xylene andmethyl acetate (MA). These second solvents may be employed singly or ina combination of two or more. More desirably, this second solvent shouldbe selected from those having a donor number of 16.5 or less. Theviscosity of this second solvent should preferably be 28 cps or less at25° C. The mixing ratio of the aforementioned ethylene carbonate in themixed solvent should preferably be 10 to 80% by volume. If the mixingratio of the ethylene carbonate falls outside this range, theconductivity of the solvent may be lowered or the solvent tends to bemore easily decomposed, thereby deteriorating the charge/dischargeefficiency. More preferable mixing ratio of the ethylene carbonate is 20to 75% by volume. When the mixing ratio of ethylene carbonate in anon-aqueous solvent is increased to 20% by volume or more, the solvatingeffect of ethylene carbonate to lithium ions will be facilitated and thesolvent decomposition-inhibiting effect thereof can be improved.

EXAMPLE Example 1 Cathodic Electrochemical Modification of Soft Carbon

40 ml of DMSO and 10 ml of deionized water were mixed and 1 g of NaClwas added to the mixed solvent to make an electrolytic solution. 10 g ofsoft carbon powder with the average diameter of about 10 μm were soakedin this electrolytic solution and a Pt wire as a working electrode wasimmersed so as to be contacted with the soft carbon powder. As a counterelectrode, a graphite rod was inserted in the electrolytic solutionwithout contacting the soft carbon powder. Negative voltage of −10V wasapplied to the working electrode to perform a cathodic electrochemicalmodification. The resultant product (sample 1) was filtered out, washedwith deionized water for many times and dried in vacuum oven. FIGS. 2Aand 2B show scanning electron microscope (SEM) images ofelectrochemically modified soft carbon particles (sample 1).

Example 2 Heat Treatment of Sample 1

Sample 1 prepared in Example 1 was further heat treated at 800° C. for 3hours in an inert atmosphere. The resultant product (sample 2) was usedfor example 2.

Comparative Example 1 Conventional Soft Carbon

Soft carbon particles (comparative sample 1) with an average particlesize of 10 pm, made from pitch cokes, is used for comparative example 1.FIGS. 3A and 3B show scanning electron microscope (SEM) images ofunmodified soft carbon particles (comparative sample 1).

Fabrication of a Test Cell

Each sample prepared in Examples 1 to 2 and Comparative example 1,carbon black, and PVDF were mixed in a weight ratio of 92:1:7 and theresultant mixture was dispersed in N-methylpyrrolidone (NMP) to preparea negative slurry.

The negative slurry was coated on a Cu foil as a current collector,dried at 120° C. for 15 min, pressed to 45 μm thick with a load of 50g/m² and cut into 22×25 mm to prepare a negative electrode. The negativeelectrode as a working electrode and a metal lithium foil as a counterelectrode were stacked by interposing porous polypropylene filmtherebetween as a separator. The resultant stack and an electrolyteprepared by dissolving 1M LiPF₆ in a mixed solvent of diethyl carbonate(DEC) and ethylene carbonate (EC) in a volume ratio of 1:1 were sealedinto an aluminum laminate container to fabricate a test cell.

The test cell was evaluated in initial charge capacity, efficiency andrate capability of 1C charge/0.1C discharge. Results are shown inTable 1. In FIG. 4, rate capability comparison of electrochemicallymodified soft carbon (sample 1), electrochemical modified soft carbonwith 800° C. 3 h heat treatment (sample 2) and soft carbon raw materials(comparative sample 1) is shown.

TABLE 1 Capacity Efficiency Rate capability (mAh/g) (%) 1 C/0.1 C SoftExample 1 237.5 86.4 66.7% carbon Example 2 224.6 83.3 63.1% base Comp.Ex. 1 209.0 85.2 59.4%

While the invention has been particularly shown and described withreference to exemplary embodiments thereof, the invention is not limitedto these embodiments. It will be understood by those of ordinary skillin the art that various changes in form and details may be made thereinwithout departing from the spirit and scope of the present invention asdefined by the claims.

1. An anode material for a lithium-ion battery comprising a carbonparticle having a particle size of 5 μm to 30 μm, and includingdefective portions on a surface of the carbon particle, the defectiveportions being grooves formed by cathodically exfoliating graphenelayers from the carbon particle.
 2. The anode material according toclaim 1, wherein the width of grooves is distributed between 100 nm and500 nm, inclusive.
 3. The anode material according to claim 2, whereinthe carbon particle is a soft carbon particle.
 4. The anode materialaccording to claim 1, further comprising anode active particles whichare capable of absorbing and desorbing lithium ions.
 5. The anodematerial according to claim 4, wherein the anode active particles aremetal or semi-metal particles or compound particles of at least oneelement selected from a group consisting of Si, Sn, Al, Ge and Pb. 6.The anode material according to claim 4, wherein the anode activeparticles adhere on the carbon particle.
 7. The anode material accordingto claim 6, wherein the carbon particle and the anode active particlesare composited with amorphous carbon layer.
 8. A method for preparing ananode material comprising: soaking carbon particles having a particlesize of 5 μm to 30 μm into an electrolytic solution, the electrolyticsolution containing a cation intercalatable between graphene layers inthe carbon particles; cathodically exfoliating graphene layers from thecarbon particles to form defective portions on surfaces of the carbonparticles; and collecting the carbon particles after the cathodicallyexfoliating.
 9. The method according to claim 8, further comprising heattreating the collected carbon particles under inert atmosphere.
 10. Themethod according to claim 8, wherein the cation is a solvate of alkalinemetal cation with an organic solvent.
 11. The method according to claim10, wherein the cation is a solvate of sodium ion with dimethylsulfoxide.
 12. A lithium ion battery comprising the anode materialaccording to claim
 1. 13. A lithium ion battery comprising the anodematerial according to claim
 2. 14. A lithium ion battery comprising theanode material according to claim
 3. 15. A lithium ion batterycomprising the anode material according to claim
 4. 16. A lithium ionbattery comprising the anode material according to claim
 5. 17. Alithium ion battery comprising the anode material according to claim 6.18. A lithium ion battery comprising the anode material according toclaim 7.